The present invention relates to mixed lanthanid-magnesium aluminates obtained in the form of monocrystals and having applications in the field of microlasers for integrated optics, optical fibre telecommunications or medicine (microsurgery, skin treatment), in the field of power lasers emitting in the infrared or visible ranges with a certain degree of tunability and power lasers tunable in the visible range more particularly making it possible to carry out treatments of materials (welding, drilling, marking, surface treatment), photochemical reactions, controlled thermonuclear fusion or polarization of the atoms of a gas, such as helium.
Certain lasers according to the invention emit at wavelengths of 1054, 1082 and 1320 nm with a certain degree of tunability, whereas other lasers emit in the range 1.5 to 3 micrometers and yet other lasers with tunability between 680 and 800 nm. Known mixed lanthanide-magnesium aluminates are in particular lanthanum-neodymium-magnesium aluminates, called LNA and of formula La.sub.1-x Nd.sub.x MgAl.sub.11 O.sub.19 with 0&lt;x.ltoreq.1 and preferably of formula La.sub.0.9 Nd.sub.0.1 MgAl.sub.11 O.sub.19. These mixed aluminates are in particular referred to in FR-A-2 448 134 and EP-A-0 043 776 filed in the name of the present Applicants. These mixed aluminates obtained in monocrystalline form have laser properties comparable to those of neodymium-doped aluminium and yttrium garnet, known under the abbreviation YAG:Nd and neodymium ultraphosphate (NdP.sub.5 O.sub.14) emitting in the infrared.
In particular, LNA has very interesting laser emission wavelengths at 1054 and 1082 nm covering that of YAG at 1064 nm. It also has another emission wavelength range around 1320 nm, which corresponds to the smallest attenuation by silica optical fibres, which thus permits the transmission of maximum information with minimum loss. LNA monocrystals can be obtained by a large number of crystallogenesis methods using a molten bath, such as the Verneuil or the Czochralski method, in order to obtain the laser effect.
However, these aluminates do not have so-called congruent fusion or melting, i.e. the monocrystal does not have the same composition as the molten bath. Thus, the production of monocrystals of these aluminates and particularly by the most widely used Czochralski method, leads to crystals having an inadequate quality (bubbles, defects), as soon as it is necessary to achieve the large sizes required by the laser industry and particularly in connection with power lasers (bar/rod of 6.35 mm diameter and 100 nm long). Moreover, LNA has only a small number of laser emission wavelengths.
Furthermore, mixed lanthanum-neodymium-magnesium aluminates are known of formula La.sub.1-x Nd.sub.x Mg.sub.y Al.sub.11 O.sub.18+y, in which x and y represent numbers such that 0&lt;x&lt;0.2 and 0&lt;y&lt;1, preferably with 0.4&lt;y&lt;0.7. These mixed aluminates in particular form the subject matter of French patent application No. 8515579, filed on behalf of the Applicants on Oct. 21, 1985. These mixed aluminates lead to an improvement compared with LNA with regards to the production of large monocrystals. However, the wavelength range covered by these lasers is still limited.